Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [107]
Among birds, long-term food storage occurs with generally northern species (Källender and Smith 1990), and in those species that exhibit no or only modest nocturnal torpor of several degrees. Food-caching behavior is found almost prominently within two families; some of the Paridae (chickadees and kin) store food for winter, and most of the Corvidae (crows, jays, magpies, nutcrackers, and ravens) do so. There is much variation within any one group. At one end of the spectrum of behavior are chickadees, which when encountering a food bonanza that they cannot eat all at once will store some of the food, stuffing it into clefts and crevices, and come back later for it. However, there is no evidence that they lay up food stockpiles for long-term use. On the other hand, European marsh tits and nuthatches may depend on stored beechnuts for a significant part of their winter diet.
A frozen apple on a white birch twig cached there by a red squirrel.
Within the crow family there is also a gradation of behavior, ranging from temporary storage of a surplus to long-term storage that sustains the animals through the winter and well into the breeding season. Like pocket mice, kangaroo rats, hamsters, and chipmunks that are adapted to carry off surplus food in their two expandable cheek pouches, corvids that cache have an expandable throat-pouch under their tongue for carrying food to storage.
The super-cachers among the Corvidae are probably the nutcrackers, the Eurasian, and the Clark’s of the mountainous regions of western North America. The first stores mainly hazelnuts. The second lives primarily on the seeds of several species of pines, such as pinyon, limber, and whitebark, which ripen in the fall (Vander Wall and Hutchins 1983). A single nutcracker routinely collects tens of thousands (and as many as 30, 000) pine seeds and stores them in 2, 500 individual caches, commonly in windswept south-facing rock ledges that may be fifteen kilometers from where the seeds were picked. Months later the birds remember the location of about 80 percent of these caches and come back to retrieve the seeds. The nutcrackers’ ability to live off the seasonal seed crop (Vander Wall and Balda 1981) depends on their astounding memory (Vander Wall and Balda 1983) for specific sites that humans would likely find impossible to match. The seed caches that are not retrieved are, in the long term, not wasted because they propogate the food source on which these birds depend.
Common ravens, when feeding on carcasses in winter, cache meat. However, their food stockpiles last only relatively short periods of time, because the caches get buried by frequent snowfalls. Additionally, meat is perishable or is easily found and dug up by carnivores with a keen sense of smell such as shrews, coyotes, and foxes. Thus, their caches are more for immediate, not long-term use.
We have only a glimpse into the general pattern of how ravens maintain an energy balance in winter, but that peek is intriguing. Ravens begin to nest in late winter, and all of the young are kicked out of the parent’s territory by late summer, if they have not left on their own already (Heinrich 1999). The young then follow the food—preferably a constant supply of fresh food such as that provided by a pack of wolves (Stahler, Heinrich, and Smith 2002). In areas of the north woods where there are no longer wolves, the ravens hunt